US police launch manhunt after six killed in Pittsburgh

US police launched a manhunt on Thursday after a brutal shooting at a backyard barbecue in Pennsylvania killed six people, including a pregnant woman and her unborn child.

Police said they were searching for at least two suspects, asking witnesses to come forward about the massacre late on Wednesday in the town of Wilkinsburg, six miles (ten kilometers) east of Pittsburgh.

Three women and a man were killed at the scene and another woman died at the hospital, Allegheny County police said in a statement.

One of the women killed was eight months pregnant, Superintendent Charles Moffatt later said at a news conference.

The medical examiner's office "officially determined that the eight-month-old fetus was also a homicide," he said. "So now we're up to 6 homicides at this point."

Most of the victims suffered multiple wounds, Moffatt said.

Police are investigating in Pennsylvania. Photo: BBC

The injured were "still in a very critical condition", he said, adding that they were cooperating with police, who had interviewed them at length.

The five adults killed were African-Americans aged between 25 and 37 years.

"They are related, cousins and a set of sisters," Moffatt said.

The first shots came from an alley next to the house where the barbecue was taking place. As the partygoers tried to flee inside, another gunman fired shots at the back porch.

There was no return fire, Lt. Andrew Schurman told AFP.

The suspects arrived and left on foot, police said.

Neighbors said they heard between 30 and 40 shots, local KDKA television reported.

Police don't have any firm suspects, Moffatt said. "We don't have enough at this time to make any arrests."

However, evidence at the crime scene pointed to two shooters who used an AK47 automatic rifle and a .9 millimeter handgun, he said.

Police did not recover the weapons but found 48 bullet casings at the scene.

"It appears it was targeted," Moffatt said of the shooters' motives. "They knew where they were going."

Seven ambulances were called to the scene and a large police force deployed.

"This street is always quiet, local resident Kayla Alexander told WPXI television. "There is nothing but kids on this street."

Police are considering drugs as a possible motive, however "everything is on the table", Moffatt said.

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) is offering a $20,000 reward for information relating to the shooting, the agency's Chris Taylor said.

Mass shootings in which at least four people are killed or injured take place almost daily in the United States, with 330 deaths recorded last year.

Firearms kill a total of 30,000 people each year, with each bloody attack reviving Americans' debate about the right to bear arms.

However, Republican lawmakers, many of whom are backed by the powerful lobby group the National Rifle Association, have blocked President Barack Obama's attempt to pass gun control legislation. - AFP